The present invention relates to a final depository for radioactive wastes in a salt stock or other equivalent geological formation, in which the wastes are housed in storable shielded containers and are retained in a section of the salt stock or equivalent formation.
All over the world, the storage of radioactive wastes in geological formations is considered to be the best method of keeping these materials, which will constitute a danger over millenia, safely away from inhabited regions of the earth. This exclusion from the biosphere must be effective until the radioactivity has decayed to harmless levels. Depending on the composition and type of the wastes, this requires time periods from a hundred years to several hundred thousand years.
When these wastes are stored in geological formations, flooding of the final depository with water leads to subsequent penetration of leached radioactivity into the groundwater, which constitutes the most dangerous path of propagation.
Attempts at solving this problem so far have been directed toward protecting the radioactive wastes as well as possible against leaching by water or aqueous solutions of natural salts. This is done with the use of high quality fixing materials such as glass, ceramic, bitumen, plastics and hydraulic binders. Additionally, storage chambers are sealed by means of salt and hydraulic binders and the interstices between waste barrels are filled with loose salt. Wastes introduced into bores are protected by plugs of various materials such as salt, cement or bitumen.
In all these cases, however, there must be considered, as constituting the greatest possible danger, a break-in of water in which part or all of the stored wastes come into direct contact with the penetrating aqueous solutions. It is then likely that a fraction of the stored radioactive material is leached out and is distributed, by convection and diffusion, over a more or less large area of the underground cavities. Under favorable conditions it is not impossible that parts of the contaminated salt solutions also come into contact, after a shorter or longer period of time, with the groundwater to be found at a shallower depth. This case would jeopardize and endanger the intended goal of final storage, i.e. long-term secure exclusion of the radioactivity from the biosphere.
Highly radioactive, heat developing wastes are stored in final depositories provided especially for this purpose which are designed according to the principle of conventional underground mines. The highly radioactive wastes contain approximately 99% of the activity that has to be permanently disposed of. Wastes which are embedded in a glass matrix are packed in stainless steel containers and permanently stored in bore holes.
There is presently no known technique for preventing, with certainty, wastes from coming into contact with water during or after their storage. According to the prior art concepts, connections with the level of the final depository are made horizontally and directly with air shafts via a tap at the same depth. Also under consideration is the possibility of the final depository filling with water during or after storage although there is only a very slight probability of such an event occurring. The direct, horizontal connection of the bottom of the final depository with the air shafts according to prior art concepts would favor propagation of the radioactivity due to convection.
It must be assumed that a water break-in, if it occurs at all, would take place through a tubular shaft into the area of covering layers containing groundwater. The tubular shaft is generally considered the most vulnerable portion of a final depository since here the water flows over its shortest path through groundwater conductors and the geological storage formation, e.g. rock salt. For that reason the greatest attention should be accorded in the future to a shaft design that is as permanently completely watertight as possible in such areas. If water should nevertheless break in, such water would flow to the lowest point in the shaft and flow from there into the horizontally branching bottom of the depository or portions of the depository.
The generation of heat in the highly radioactive wastes locally heats up the enclosing geologic formation. The salt solution which is under hydrostatic pressure could then penetrate the storage bores, attack the hot waste containers and leach out radioactivity. The high temperatures involved could also produce strong convection currents in the vertical bore holes. Differences in density then would cause the leachable activity to be distributed in a very short time over the entire horizontal portion of the final depository.
Due to the higher temperature of the salt solutions in the lowest portion of the final depository, movement of the radioactive nuclides will also be able to take place in a vertical direction through the tubular shafts of the final depository. It is thus possible that the radioactivity finally propagates to the level of the water entrance point and thus into the groundwater.
Active countermeasures to protect the stored wastes against contact with salt solutions do not exist in the present state of the art and are not conceivable with final depositories of prior art design.